Emission treatment system with NSR and SCR catalysts

ABSTRACT

Provided is an emissions treatment system for an exhaust stream, having a NOx storage reduction (NSR) catalyst with a NOx sorbent at a concentration of at least 0.1 g/in 3  and a platinum group metal component dispersed on a refractory metal oxide support; and, an SCR catalyst disposed downstream of the NSR catalyst. The emissions treatment system is advantageously used for the treatment of exhaust streams from diesel engines and lean burn gasoline engines.

STATEMENT OF RELATED CASES

This application is a continuation of applicant's U.S. patentapplication Ser. No. 10/975,428, filed Oct. 29, 2004, which claimspriority from U.S. Provisional Patent Application Ser. No. 60/517,137,filed Nov. 4, 2003.

The present invention relates to emissions treatment systems having NSRand SCR catalysts, and methods useful for reducing contaminants inexhaust gas streams, especially exhaust gas streams containing nitrogenoxides (NOx). More specifically, the present invention is concerned withimproved NSR catalysts, emissions treatment systems and methods fortheir use with lean burn engines, including diesel engines and lean burngasoline engines.

Operation of lean burn engines, e.g., diesel engines and lean burngasoline engines, provide the user with excellent fuel economy, and havevery low emissions of gas phase hydrocarbons and carbon monoxide due totheir operation at high air/fuel ratios under fuel lean conditions.Diesel engines, in particular, also offer significant advantages overgasoline engines in terms of their durability, and their ability togenerate high torque at low speed. However, exhaust from lean burngasoline engines is characterized by relatively high emissions of NOx ascompared to conventional gasoline engines that operate at or close tostoichiometric air/fuel conditions. Effective abatement of NOx from leanburn engines is difficult to achieve because high NOx conversion ratestypically require reductant-rich conditions. Conversion of the NOxcomponent of exhaust streams to innocuous components generally requiresspecialized NOx abatement strategies for operation under fuel leanconditions.

One such strategy for the abatement of NOx in the exhaust stream fromlean burn engines uses NOx storage reduction (NSR) catalysts, which arealso known in the art as “NOx traps.” NSR catalysts contain NOx sorbentmaterials capable of adsorbing or “trapping” oxides of nitrogen underlean conditions and platinum group metal components to provide thecatalyst with oxidation and reduction functions. In operation, the NSRcatalyst promotes a series of elementary steps which are depicted belowin Equations 1-5. In an oxidizing environment, NO is oxidized to NO₂(Equation 1), which is an important step for NOx storage. At lowtemperatures, this reaction is typically catalyzed by the platinum groupmetal component, e.g., a platinum component. The oxidation process doesnot stop here. Further oxidation of NO₂ to nitrate, with incorporationof an atomic oxygen, is also a catalyzed reaction (Equation 2). There islittle nitrate formation in absence of the platinum group metalcomponent even when NO₂ is used as the NOx source. The platinum groupmetal component has the dual functions of oxidation and reduction. Forits reduction role, the platinum group metal component first catalyzesthe release of NOx upon introduction of a reductant, e.g., CO (carbonmonoxide) or HC (hydrocarbon) (Equation 3) to the exhaust. This step mayrecover some NOx storage sites but does not contribute to any reductionof NOx species. The released NOx is then further reduced to gaseous N₂in a rich environment (Equations 4 and 5). NOx release can be induced byfuel injection even in a net oxidizing environment. However, theefficient reduction of released NOx by CO requires rich conditions. Atemperature surge can also trigger NOx release because metal nitrate isless stable at higher temperatures. NOx trap catalysis is a cyclicoperation. Metal compounds are believed to undergo a carbonate/nitrateconversion, as a dominant path, during lean/rich operations.

Oxidation of NO to NO₂NO+1/2O₂→NO₂   (1)

NOx Storage as Nitrate2NO₂+MCO₃+1/2O₂→M(NO₃)₂+CO₂   (2)

NOx ReleaseM(NO₃)₂+2CO→MCO₃+NO₂+NO+CO₂   (3)

NOx Reduction to N₂NO₂+CO→NO+CO₂   (4)2NO+2CO→N₂+2CO₂   (5)

In Equations 2 and 3, M represents a divalent metal cation. M can alsobe a monovalent or trivalent metal compound in which case the equationsneed to be rebalanced.

While the reduction of NO and NO₂ to N₂ occurs in the presence of theNSR catalyst during the rich period, it has been observed that ammonia(NH₃) can also form as a by-product of a rich pulse regeneration of theNSR catalyst. For example, the reduction of NO with CO and H₂O is shownbelow in equation (6).

Reduction of NO to NH₃2NO+5CO+3H₂O→2NH₃+5CO₂   (6)

This property of the NSR catalyst mandates that NH₃, which is itself anoxious component, must also now be converted to an innocuous speciesbefore the exhaust is vented to the atmosphere.

An alternative strategy for the abatement of NOx under development ofmobile applications (including treating exhaust from lean burn engines)uses selective catalytic reduction (SCR) catalyst technology. Thestrategy has been proven effective as applied to stationary sources,e.g., treatment of flue gases. In this strategy, NOx is reduced with areductant, e.g., NH₃, to nitrogen (N₂) over an SCR catalyst that istypically composed of base metals. This technology is capable of NOxreduction greater than 90%, thus it represents one of the bestapproaches for achieving aggressive NOx reduction goals.

Ammonia is one of the most effective reductants for NOx at leancondition using SCR technologies. One of the approaches beinginvestigated for abating NOx in diesel engines (mostly heavy duty dieselvehicles) utilizes urea as a reductant. Urea, which upon hydrolysisproduces ammonia, is injected into the exhaust in front of an SCRcatalyst in the temperature range 200-600° C. One of the majordisadvantages for this technology is the need for an extra largereservoir to house the urea on board the vehicle. Another significantconcern is the commitment of operators of these vehicles to replenishthe reservoirs with urea as needed, and the requirement of aninfrastructure for supplying urea to the operators. Therefore, lessburdensome and alternative sources for supplying the reductant NH₃ forthe SCR treatment of exhaust gases are desirable.

Emissions treatment systems that utilize the catalytic reduction of NOxin the exhaust to generate NH₃, in place of an external reservoir of NH₃or NH₃ precursor are known in the art. In other words, a portion of theNOx component of the exhaust is used as an NH₃ precursor in suchsystems. For instance, U.S. Pat. No. 6,176,079 discloses a method fortreating an exhaust gas from a combustion system that is operatedalternately in lean and rich conditions. In the method, nitrogen oxidesare intermediately stored during lean operation, and released duringrich operation to form NH₃ that is stored. The stored NH₃ can bereleased, and thereby reduce nitrogen oxides during a subsequent leanoperation.

European Patent Publication No. 773 354 describes a device for purifyingthe exhaust gas of an engine that has a three way catalyst connected toan NH₃ adsorbing and oxidizing (NH₃-AO) catalyst. The engine is operatedwith alternating rich and lean periods. During a rich operation thethree way catalyst synthesizes NH₃ from NOx in the inflowing exhaustgas, and the NH₃ is then adsorbed on the NH₃-AO catalyst. During thelean operation NOx passes through the three way catalyst and theadsorbed NH₃ is desorbed and reduces the inflowing NOx.

International Published Patent Application WO 97/17532 discloses amethod and device for purifying the exhaust gas from an engine, and inparticular, describes a method and device for purifying NOx in theexhaust gas. In one embodiment, the publication describes a device thathas a three way catalyst upstream of, and on the same carrier as a NOxoccluding and reducing catalyst. Downstream of the NOx occluding andreducing (NOx-OR) catalyst is a NH₃ adsorbing and oxidation (NH₃-AO)catalyst. To prevent any NH₃ breakthrough, the device is also equippedwith a NH₃ purifying catalyst downstream of the NH₃-AO catalyst. Theair/fuel ratio of the cylinders of the engine are alternately andrepeatedly rendered lean and rich to thereby render the exhaust gasair/fuel ratio, alternately and repeatedly rich and lean.

In the method described for this device in the WO97/17532 publication,when the air/fuel ratio of the exhaust gas is lean, NOx passes throughthe three way catalyst, and NOx is occluded in the NOx-OR catalyst. Itis described that any NOx passing through the NOx-OR catalyst ispurified in the following NH₃-AO catalyst. NH₃ is desorbed from theNH₃-AO catalyst when the air/fuel ratio of the exhaust gas is lean, andthe desorbed NH₃ reduces the NOx.

When the air/fuel ratio of the exhaust gas is rich, a part of the NOx inthe exhaust gas is converted to NH₃ in the three way catalyst. The NH₃then passes into the NOx-OR catalyst, where the NOx is released, reducedand purified by the inflowing NH₃. Any NH₃ passing through the NOx-ORcatalyst that is not consumed by the reduction of NOx is adsorbed on theNH₃-AO catalyst, or is purified further downstream in the NH₃ purifyingcatalyst.

A problem associated with methods that utilize a portion of the NOx inthe exhaust gas as an NH₃ precursor is that, depending on operatingconditions, insufficient NH₃ is synthesized during rich operatingperiods to treat the NOx during lean periods (i.e., when the exhaust gascomposition has a λ>1). For instance, in systems that use a three waycatalyst to convert NOx to NH₃, the maximum amount of NH₃ that is formedduring a rich period (i.e., when the exhaust gas composition has a λ<1)is limited by the amount of the NOx component in the exhaust gas.Moreover, when the exhaust gas is rich, the amount of NOx in the exhaustis typically lower than the amount when the exhaust gas is lean.Therefore, less NH₃ precursor is available for forming NH₃ to treat ahigher concentration of NOx that passes through the treatment system ina subsequent lean exhaust period. This deficiency can limit the range ofoperating conditions where NOx can be effectively treated by theemissions treatment system. Systems that have the capacity to form moreNH₃ during rich cycles would therefore provide the potential foraccommodating a broader range of operating conditions where NOx can beeffectively treated.

As the conditions that emission treatment systems operate under vary fordifferent vehicles powered by lean burn engines, flexible approaches forthe design of emission treatment systems are needed to achieve ever morestringent requirements for NOx abatement. In particular, approaches thataccount for the effect on NOx storage and NH₃ formation during lean andrich periods of operation of altering the NSR catalyst composition offermore reliable and practical pathways to achieving this goal.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to an emissions treatment systemfor an exhaust stream from, for example, a diesel engine or a lean burngasoline engine. The system has a NOx storage reduction (NSR) catalystwith a NOx sorbent at a concentration of at least 0.1 g/in³ and aplatinum group metal component dispersed on a refractory metal oxidesupport. The emissions treatment system also has an SCR catalystdisposed downstream of the NSR catalyst.

The emissions treatment system typically has a controller toperiodically lower the air/fuel ratio in the exhaust stream upstream ofthe NSR catalyst. In some embodiments, the controller is an enginemanagement system that periodically operates the engine in a rich mode.The controller may also comprise an injector that periodically meters areducing agent selected from at least one of a hydrocarbon fuel, carbonmonoxide and hydrogen into the exhaust stream upstream of the NSRcatalyst to form a rich gaseous stream.

The platinum group metal components of the NSR catalyst are generallyselected from the group consisting of platinum, palladium, rhodiumcomponents and mixtures thereof, and are typically present at from 10 to250 g/ft³. Preferably, the NSR catalyst has from 50 to 200 g/ft³ of aplatinum component.

The NOx sorbent of the NSR catalyst is at least one alkali or alkalineearth metal oxide selected from the oxides of lithium, sodium,potassium, cesium, magnesium, calcium, strontium and barium. Preferablythe NOx sorbent includes barium.

The NSR catalyst of the emissions treatment system may further containan oxygen storage component preferably selected from one or more oxidesof lanthanum, cerium, praseodymium and neodymium. If present, there isless than 0.5 g/in³ of the oxygen storage component present in the NSRcatalyst. The oxygen storage component helps to remove sulfur from theNSR catalyst during rich periods and helps to reduce the size and amountparticulate matter at the soot filter.

The refractory metal oxide of the NSR catalyst in the emissionstreatment system is typically selected from the group consisting ofalumina, titania, zirconia, zeolites, ceria dispersed on alumina,titania dispersed on alumina and silica on alumina. In a preferredembodiment, the refractory metal oxide of the NSR catalyst is aluminahaving a surface area of at least 50 m²/g.

In some preferred embodiments of the emissions treatment system, the NSRcatalyst has at least two catalyst layers deposited on a substrate. Inother embodiments, the NSR catalyst is a single catalyst layer depositedon a substrate. The substrate can be, for example, a honeycomb flowthrough substrate or wall flow filter substrate.

One preferred SCR catalyst for the emissions treatment system contains azeolite component, for instance, an iron- or copper-exchanged zeolite.Another preferred SCR catalyst contains V₂O₅, WO₃ and TiO₂. The SCRcatalyst can be deposited on a ceramic or metallic honeycomb flowthrough substrate. An alternative embodiments, the SCR catalyst may bedisposed on a wall flow filter substrate.

In an optional embodiment, the emissions treatment system also has adiesel oxidation catalyst either upstream of the NSR catalyst ordownstream of the SCR catalyst. The diesel oxidation catalyst may be,for example, disposed on a soot filter either upstream of the NSRcatalyst or downstream of the SCR catalyst.

A preferred embodiment of the invention relates to an emissionstreatment system for an exhaust stream that has a NOx storage reduction(NSR) catalyst with a barium component at a concentration of at least0.2 to 0.6 g/in³ and a platinum metal component at a concentration of 50to 200 gift³ dispersed on a refractory metal oxide support. This systemalso has an SCR catalyst disposed downstream of the NSR catalyst,wherein the SCR catalyst comprises an iron-exchanged zeolite material.

In another aspect, the invention relates to a method for converting NOxin an exhaust gas from a diesel engine or lean burn gasoline engine toN₂. The method includes:

(a) contacting the exhaust gas comprising NOx in a lean period with aNOx storage reduction (NSR) catalyst comprising a platinum group metalcomponent and at least 0.1 g/in³ of a NOx sorbent dispersed on arefractory metal oxide to sorb a portion of NOx from the exhaust gas andallowing a portion of the NOx to flow through the NSR catalystuntreated;

(b) periodically altering the air/fuel ratio of the exhaust stream toprovide a rich gaseous stream during a rich period;

(c) contacting the rich gaseous stream with the NSR catalyst to reducethe sorbed NOx to NH₃; and,

(d) subsequently contacting a mixture of the NH₃ and the untreated NOxin the exhaust gas with an SCR catalyst to form N₂.

The method preferably further includes step (c1), which follows step(c). Step (c1) is sorbing a portion of the NH₃ on the SCR catalyst.

The alteration of the air/fuel ratio of the exhaust gas stream in (b),may be conducted by periodically operating the engine in a rich mode orby injecting a hydrocarbon fuel into the exhaust gas upstream of the NSRcatalyst.

The exhaust gas of (a) generally has a space velocity of 10,000 to200,000 h⁻¹ through the NSR catalyst. Similarly, the exhaust gas of (d)generally has a space velocity of 10,000 to 200,000 h⁻¹ through the SCRcatalyst.

Preferably, the λ of the rich gaseous stream is from 0.80 to 0.995.During lean periods the λ of the exhaust gas is preferably greater than1.1. Typically, the rich period is from 1 to 50% of the lean period.

Generally the exhaust gas of (a) has a temperature of 100 to 600° C.through the NSR catalyst. Preferably, the exhaust gas of (a) has atemperature of 150 to 450° C. through the NSR catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram showing one embodiment of the emissionstreatment system having NSR and SCR catalysts.

FIG. 1B is a block diagram showing one embodiment of an emissionstreatment system having DOC, NSR and SCR catalysts.

FIG. 2 shows the total NOx conversion on System I as a function of COconcentration in a rich pulse.

FIG. 3 shows the NH₃ formation on System I as a function of COconcentration in a rich pulse.

FIG. 4 shows the N₂ formation on System I as a function of COconcentration in a rich pulse.

FIG. 5 shows the total NOx conversion on System II as a function of COconcentration in a rich pulse.

FIG. 6 shows the NH₃ formation on System II as a function of COconcentration in a rich pulse.

FIG. 7 shows the N₂ formation on System II as a function of COconcentration in a rich pulse.

FIG. 8 shows the NH₃ formation on System I as a function of rich timing.

FIG. 9 shows the N₂ formation on System I as a function of rich timing.

FIG. 10 shows the NH₃ formation on System II as a function of richtiming.

FIG. 11 shows the N₂ formation on System II as a function of richtiming.

FIG. 12 shows the NH₃ formation over three NSR catalysts, Catalysts A, Band C.

FIG. 13 shows a schematic view in perspective of one embodiment of anemissions treatment system.

FIG. 14 shows a partial, sectional section view of one embodiment of anemissions treatment system.

DEFINITIONS

The following terms shall have, for the purposes of this application,the respective meanings set forth below.

“Lean gaseous streams” including lean exhaust streams mean gas streamsthat have a λ>1.0.

“Lean periods” refer to periods of exhaust treatment where the exhaustgas composition is lean, i.e., has a λ>1.0.

“Platinum group metal components” refer to platinum group metals or oneof their oxides.

“Rare earth metal components” refer to one or more oxides of thelanthanum series defined in the Periodic Table of Elements, includinglanthanum, cerium, praseodymium and neodymium.

“Rich gaseous streams” including rich exhaust streams mean gas streamsthat have a λ<1.0.

“Rich periods” refer to periods of exhaust treatment where the exhaustgas composition is rich, i.e., has a λ<1.0.

“Washcoat” has its usual meaning in the art of a thin, adherent coatingof a catalytic or other material applied to a refractory substrate, suchas a honeycomb flow through monolith substrate, which is sufficientlyporous to permit the passage there through of the gas stream beingtreated.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an emissions treatment system effective for thetreatment of the components of exhaust gases from lean burn enginesincluding unburned gaseous hydrocarbons, can monoxide, particulatematter (e.g., in diesel exhaust) and especially, NOx. The system has anupstream NSR catalyst with dual catalytic function and a downstream SCRcatalyst, and is operated with alternating lean and rich exhaust gases.The NSR catalyst promotes the storage of NOx during a lean period ofoperation according to equations (1) and (2), and during a rich periodit catalyzes not only the reduction of stored NOx to N₂ (equation 5),but also the formation of NH₃ from both gaseous NOx and stored NOx(equation 6). Applicants have recognized that through selection of NSRcatalyst components and their respective concentrations in the NSRcatalyst compositions, sufficient and predictable quantities of NH₃ areformed when the exhaust gas is rendered rich to effectively treat NOxwhen the exhaust gas is lean. The approaches described herein offer aflexible, efficient and predictable approach for designing emissionstreatment systems capable of accommodating exhaust gases emitted from avariety of lean burn engines, including diesel engines and lean burngasoline engines.

One embodiment of the emissions treatment system, denoted as 10A in FIG.1A, includes an NSR catalyst that is upstream of an SCR catalyst. TheNSR catalyst can be in the form of an NSR catalyst composition on arefractory ceramic or metallic substrate. In a lean period, an exhaustgas composition flowing from the exhaust manifold of an engine 1contacts the NSR catalyst 2 where a portion of the NOx contained thereinis adsorbed on to the NSR catalyst, and another portion of the NOxpasses through NSR catalyst untreated. The air/fuel ratio of the exhaustgas composition is then altered to render a rich gaseous stream during arich period. The rich gaseous stream then contacts the NSR catalyst toreduce the sorbed NOx to N₂ and NH₃. The amount of NH₃ so produced cantherefore be greater than the engine out NOx emission. In addition,gaseous NOx in the rich stream is also reduced to provide furtherquantities of NH₃. The NH₃ then passes downstream in the system to anSCR catalyst 3. The SCR catalyst can be in the form of an SCR catalystcomposition disposed on a refractory ceramic or metallic substrate.Typically, a portion of the NH₃ is adsorbed on to the SCR catalyst. Theuntreated NOx then contacts the SCR catalyst and reacts with the storedNH₃ to form N₂ during the lean period.

In alternative embodiments, the NSR and SCR catalysts may be disposed inseparate zones on the same substrate, where the NSR catalyst is disposedon the upstream segment of the substrate, and the SCR catalyst isdisposed on the downstream segment.

The use of the NSR catalysts described herein provides a significantadvantage for the design of emissions treatment systems for lean burnengines. As the NSR catalyst has both a NOx storage function during leanperiods of operation and an NH₃ generating function during rich periodsof operation, inclusion of separate catalyst substrates to perform thesetwo functions is unnecessary. As a consequence, the burden of preparingand housing separate catalyst substrates is absent. Moreover, overallplatinum group metal usage is diminished with the dual function NSRcatalyst; since catalysts that promote NOx storage and catalysts thatpromote NH₃ formation both generally have platinum group metalcomponents in their compositions. Emissions treatment systems that havea single NSR catalyst instead of separate catalysts for NOx storage andNH₃ formation therefore can afford the system designer with significantcost savings.

The air/fuel ratio of the exhaust gas composition may be altered toprovide a rich gaseous stream by a number of methods known to those ofskill in the art. Controllers that periodically operate the lean burnengine in a rich mode, or more directly alter the air/fuel ratio of theexhaust stream can be used. For instance, the air/fuel ratio can be maderich by periodically operating the engine in a rich mode using wellknown engine management controls. Alternatively, the exhaust gas streammay be rendered rich by periodically metering a hydrocarbon fuel (e.g.,diesel fuel) upstream of the NSR catalyst. A rich gaseous exhaust streammay also be formed by adding CO and H₂ to the exhaust upstream of theNSR catalyst, which may be generated, for example, by treatment of asmall quantity of hydrocarbon fuel in a partial oxidation reaction.

The amount of NH₃ produced during a rich period depends both on thelength and intensity of the rich pulse use to generate the rich gaseousstream. For purposes of operating the emissions treatment system of theinvention during a rich period, the rich gaseous stream generally has aλ of from 0.80 to 0.995. Preferably, the rich gaseous stream has a λ offrom 0.90 to 0.95. During a lean period, the lean gaseous streampreferably has a λ>1.1. The length of the rich period is generally 1 to50% of the lean period. More preferably, the length of the rich periodis 2 to 10% of the lean period. Such operating parameters ensure thatadequate levels of NH₃ are generated with minimum fuel penalty.

Space velocities for treating NOx with the inventive emissions treatmentsystem through the NSR catalyst are from 10,000 to 200,000 h⁻¹. Morepreferably, the space velocity of the exhaust gas is from 10,000 to100,000 h⁻¹. Similarly, the space velocities of the exhaust gas throughthe SCR catalyst are preferably from 10,000 to 200,000 h⁻¹, and morepreferably, from 10,000 to 100,000 h⁻¹.

NSR Catalyst Composition

The NSR catalyst composition used in the inventive system contains a NOxsorbent and a platinum group metal component dispersed on a refractorymetal oxide support. In addition, the NSR composition optionallycontains other components such as oxygen storage components and ironcomponents that significantly affect the quantity of NH₃ formed during arich period of operation. Such compositions are similar to thosedisclosed in co-pending U.S. patent application Ser. No. 10/355,779,filed Jan. 31, 2003 which is incorporated herein by reference. Such NSRcompositions exhibit good NOx storage/ NOx reduction activity at exhausttemperature of 100 to 600° C., and more particularly, at temperatures of150 to 450° C. In addition, such NSR catalyst compositions exhibitoutstanding thermal stability and the ability to remove sulfur compoundsunder moderate conditions.

The NSR catalyst may take any form such as self-supporting particles(e.g., tablets, beads) or as a solid honeycomb monolith formed of thecatalyst composition. However, the NSR catalyst composition ispreferably disposed as a washcoat or as a combination of washcoats (toform a layered catalyst composite) on a ceramic or metallic substrate.In preferred embodiments of the invention the NSR catalyst is either inthe form of a single layer, or a bi-layer catalyst composite with thebottom layer adhered to the substrate and the top layer overlying to thebottom layer.

The support for the NSR catalyst composition is composed of a highsurface area refractory metal oxide such as alumina, titania, zirconia;mixtures of alumina with one or more of titania, zirconia and ceria;ceria coated on alumina or titania coated on alumina. The refractorymetal oxide may consist of, or contain a mixed oxide such assilica-alumina, aluminosilicates which may be amorphous or crystalline,alumina-zirconia, alumina-chromia, alumina-ceria and the like. Thepreferred refractory metal oxides are gamma alumina, ceria coated onalumina and titania coated on alumina.

Typically, the refractory metal oxide will have a specific surface areaof about 50 to about 300 m²/g. The support is typically present in anamount of about 0.5 to about 3.0 g/in³, this amount being the totalamount in embodiments with two or more catalyst layers. In embodimentsof the invention where there are two layers, the support chosen for thebottom layer need not be, but is conveniently, the same as that chosenfor the top layer. Moreover, the amount of the support in the bottomlayer need not be the same as that in the top layer, so long as theamounts of the supports in the bottom and top layers are within theforegoing range.

Dispersed on the refractory metal oxide support will be one or moreplatinum group metal components, e.g., platinum, palladium, rhodium andmixtures thereof; preferably, the precious metal component comprisesplatinum. These components of the NSR catalyst promote oxidation andreduction of nitrogen species. The amount of loading of the platinumgroup metal component will be in the range of about 10 to about 250g/ft³, and preferably, the amount will be in the range of 50 to 200g/ft³, these amounts being the total amount in embodiments with two ormore catalyst layers. Here again, in embodiments of the invention wherea bi-layer catalyst composite is used, the platinum group metalcomponent chosen for the bottom layer need not be, but is conveniently,the same as that chosen for the top layer. Moreover, the amount of theplatinum group metal component in the bottom layer need not be the sameas that in the top layer, so long as the amounts of the platinum groupmetal components in the bottom and top layers are within the foregoingrange.

For the purposes of the present invention, the NSR catalyst alsocontains at least one NOx sorbent component to ensure an adequate NOxstorage capacity. In addition, the NOx storage capacity significantlyaffects the ability of the NSR catalyst to form NH₃, since the formationof NH₃ during a rich period of operation is in part limited by thesupply of stored NOx (i.e., as a metal nitrate). Typically, the NOxsorbent component is present in an amount of at least 0.1 g/in³, such asfrom 0.1 to 1.0 g/in³ to ensure adequate NOx storage. More preferablythere is at least about 0.2 g/in³ of NOx sorbent, and still morepreferably at least 0.3 g/in³ of NOx sorbent in the composition. Asuitable NOx sorbent component comprises a basic oxygenated compound ofan alkali or alkaline earth metal; the alkali metal may be lithium,sodium, potassium or cesium, and the alkaline earth metal may bemagnesium, calcium, strontium or barium. The preferred alkali metalcomprises potassium, while the preferred alkaline earth metal comprisesbarium.

In embodiments of the invention where the NSR catalyst is formed as abi-layer catalyst composite, it is preferred that the NOx sorbentcomponent in the bottom layer is present in an amount greater than about0.3 g/in³ and that the NOx sorbent in the top layer is present in theamount of 0.0 to less than about 0.3 g/in³. Amounts of the NOx sorbentcomponent in the top layer in amounts higher than about 0.3 g/in³ orgreater may have a deleterious effect on the capability of the platinumgroup metal components to catalyze the oxidation of hydrocarbons andcarbon monoxide to carbon dioxide.

An optional component of the NSR catalyst composition that affects thequantity of NH₃ that forms are oxygen storage components which typicallyare formed from rare earth metal components. While oxygen storagecomponents improve the desulfation capacity of the composition (whichmay be important for some diesel applications), too great aconcentration of the oxygen storage composition limits the quantity ofNH₃ that can be formed during rich operation. While not wishing to bebound to any specific theory, Applicants believe that the presence ofoxidized species of the oxygen storage component results in oxidation ofNH₃ to NOx or N₂O during a rich period, thereby limiting the quantity ofNH₃ that is ultimately formed by the NSR catalyst. The amount of theoxygen storage component is therefore less than 0.5 g/in³ andpreferably, less than 0.35 g/in³.

The oxygen storage component contains at least one oxide of a metalselected from the group consisting of rare earth metal components andmost preferably a cerium or praseodymium component, with the mostpreferred oxygen storage component being cerium oxide (ceria). Theoxygen storage component may be dispersed on the refractory metal oxidesupport by, for example, dispersing a soluble precursor (e.g., ceriumnitrate) on the refractory metal oxide support. Alternatively, theoxygen storage component is provided in bulk form in the composition. Bybulk form it is meant that a composition is in a solid, preferably asfine particles which can be as small as 1 to 15 microns in diameter orsmaller, as opposed to being dispersed in solution in the base metalwashcoat. When praseodymium is used, it is preferably used incombination with ceria.

In some embodiments, the oxygen storage component may be composed of abulk fine particulate material of co-formed rare earth metal-zirconiacomposite (e.g., a ceria-zirconia composite) such as are commerciallyavailable or are readily apparent to those of skill in the art. Forinstance, co-formed composites are described in U.S. Pat. No. 5,057,483.These particles do not react with stabilized alumina washcoat andmaintain a BET surface area of above 40 m²/g upon exposure to 900° C.for a long period of time. For purposes of calculating the amount ofoxygen storage component added in the NSR catalyst composition, theproportion of the rare earth component (e.g., ceria) of the compositematerial is the relevant component.

An iron component is another optional component that affects thequantity of NH₃ that is formed by the NSR catalyst during a rich period.In particular, inclusion of an iron component increases the quantity ofNH₃ formed over similar NSR compositions where an iron component isabsent. If present, the concentration of the iron component in the NSRcatalyst is typically present at 0.05 to 0.3 g/in³, and is preferablypresent at 0.1 to 0.2 g/in³.

Other components that may be added to the NSR composition include othertransition metals such as zirconium, manganese, yttrium and titanium. Ifused, such transition metal components are typically present in anamount of about 0.01 to about 0.5 g/in³.

The NSR catalyst composite of the present invention may be readilyprepared by processes well known in the prior art. A representativeprocess for preparing a bi-layer NSR catalyst is set forth below.

The catalyst composite can be readily prepared in layers on a monolithichoneycomb substrate. For the bottom layer, finely divided particles of ahigh surface area refractory metal oxide such as gamma alumina areslurried in an appropriate vehicle, e.g., water. The substrate may thenbe dipped one or more times in such slurry or the slurry may be coatedon the substrate (e.g., honeycomb flow through substrate) such thatthere will be deposited on the substrate the desired loading of themetal oxide, e.g., about 0.5 to about 3.0 g/in³. Components such as theplatinum group metals, transition metal oxides, stabilizers, promotersand the NOx sorbent component may be incorporated in the slurry as amixture of water soluble or water-dispersible compounds or complexes.Thereafter the coated substrate is calcined by heating, e.g., at 400 to600° C. for 1 to 3 hours.

Typically, the platinum group metal component, e.g., platinum component,is dispersed on the refractory metal oxide, e.g., activated alumina,using a platinum group metal salt or complex (or platinum group metalprecursor). For the purposes of the present invention, the term“platinum group metal precursor” means any compound, complex, or thelike which, upon calcination or use thereof, decomposes or otherwiseconverts to a catalytically active form, usually the metal or the metaloxide. Water-soluble compounds or water-dispersible compounds orcomplexes of the metal component may be used as long as the liquidmedium used to impregnate or deposit the metal component onto therefractory metal oxide support particles does not adversely react withthe metal or its compound or its complex or other components which maybe present in the catalyst composition and is capable of being removedfrom the metal component by volatilization or decomposition upon heatingand/or application of a vacuum. In some cases, the completion of removalof the liquid may not take place until the catalyst is placed into useand subjected to the high temperatures encountered during operation.Generally, both from the point of view of economics and environmentalaspects, aqueous solutions of soluble compounds or complexes of theplatinum-group metals are preferred. For example, suitable compounds arechloroplatinic acid, amine-solubilized platinum hydroxide, palladiumnitrate or palladium chloride, rhodium chloride, rhodium nitrate,hexamine rhodium chloride, etc. During the calcination step, or at leastduring the initial phase of use of the composite, such compounds areconverted into a catalytically active form of the metal or a compoundthereof.

A preferred method of forming the bottom layer of the layered catalystcomposite of the invention is to prepare a mixture of a solution of aplatinum group metal precursor and at least one finely divided, highsurface area, refractory metal oxide support, e.g., gamma alumina, whichis sufficiently dry to absorb substantially all of the solution to forma slurry. Preferably, the slurry is acidic, having a pH of about 2 toless than 7. The pH of the slurry may be lowered by the addition of aminor amount of an inorganic or organic acid such as hydrochloric ornitric acid, preferably acetic acid, to the slurry. Thereafter, the NOxsorbent component, and optional transition metal components, stabilizersand/or promoters may be added to the slurry.

In a particularly preferred embodiment, the slurry is thereaftercomminuted to result in substantially all of the solids having particlesizes of less than 20 microns, i.e., 1-15 microns, in an averagediameter. The comminution may be conducted in a ball mill or othersimilar equipment, and the solids content of the slurry may be, e.g.,20-60 wt. %, preferably 35-45 wt. %.

The top layer is thereafter prepared and deposited on the bottom layerof the calcined composite in a manner similar to that described above.After all coating operations have been completed, the composite is thenagain calcined by heating, e.g., at 400 to 600° C. for 1-3 hours.

SCR Catalyst

The emissions treatment system can use a number of known SCR catalyststo treat NOx downstream of the NSR catalyst. For instance, base metal(e.g., copper, iron) exchanged zeolite compositions or vanadia-basedcompositions (e.g., V₂O₅/WO₃/TiO₂/SiO₂) can be used to form the SCRcatalyst. The SCR catalyst can be in the form of self supportingcatalyst particles or as a honeycomb monolith formed of the SCR catalystcomposition. In preferred embodiments of the invention however, the NSRcatalyst composition is disposed as a washcoat or as a combination ofwashcoats on a ceramic or metallic substrate, preferably a honeycombflow through substrate.

In a preferred embodiment of the emissions treatment system the SCRcatalyst is formed from a base metal exchanged zeolite material. SuchSCR catalyst compositions are described, for example, in U.S. Pat. No.'s4,961,917 (the '917 patent) and U.S. Pat. No. 5,516,497, which are bothhereby incorporated by reference in their entirety. Compositionsdisclosed in the '917 patent include one or both of an iron and a copperpromoter present in a zeolite in an amount of from about 0.1 to 30percent by weight, preferably from about 1 to 5 percent by weight, ofthe total weight of promoter plus zeolite. In a preferred embodiment,the zeolite includes an iron component.

Zeolites used in such compositions are resistant to sulfur poisoning andsustain a high level of activity for the SCR process. These zeoliteshave pore size large enough to permit adequate movement of the reactantmolecules NO and NH₃ in to, and the product molecules N₂ and H₂O out of,the pore system in the presence of sulfur oxide molecules resulting fromshort term sulfur poisoning, and/or sulfate deposits resulting from longterm sulfur poisoning. The pore system of suitable size isinterconnected in all three crystallographic dimensions. As is wellknown to the those skilled in the zeolite art, the crystalline structureof zeolites exhibits a complex pore structure having more or lessregularly recurring connections, intersections and the like. Poreshaving a particular characteristic, such as a given dimension diameteror cross-sectional configuration, are said to be one dimensional ifthose pores do not intersect with other like pores. If the poresintersect only within a given plane with other like pores, the pores ofthat characteristic are said to be interconnected in two(crystallographic) dimensions. If the pores intersect with other likepores lying both in the same plane and in other planes, such like poresare said to be interconnected in three dimensions, i.e., to be “threedimensional.” It has been found that zeolites which are highly resistantto sulfate poisoning and provide good activity for the SCR process, andwhich retain good activity even when subject to high temperatures,hydrothermal conditions and sulfate poisons, are zeolites which havepores which exhibit a pore diameter of at least about 7 Angstroms andare interconnected in three dimensions. Without wishing to be bound byany specific theory, it is believed that the interconnection of pores ofat least 7 Angstroms diameter in three dimensions provides for goodmobility of sulfate molecules throughout the zeolite structure, therebypermitting the sulfate molecules to be released from the catalyst tofree a large number of the available adsorbent sites for reactant NOxand NH₃ molecules.

In preferred embodiments of the invention, the zeolites have a Si/Alratio of at least 2.5, and more preferably of at least 10. Any zeolitesmeeting the foregoing criteria are suitable for use in the practices ofthe present invention; specific zeolites which meet these criteria areBeta, ZSM-5, ZSM-11, dealuminated Y, and dealuminated mordenite. Otherzeolites may also satisfy the aforementioned criteria.

When deposited on the honeycomb monolith substrates, such SCR catalystcompositions are deposited at a concentration of at least 1.3 g/in³ toensure that the desired NOx reduction is achieved and to secure adequatedurability of the catalyst over extended use. In a preferred embodiment,there is at least 1.6 g/in³ of SCR composition, and in particular, thereis at least 1.6 to 2.4 g/in³ of the SCR composition disposed on the wallflow monolith.

Substrates

Preferably, each of the NSR and the SCR catalyst compositions aredisposed on a substrate. The substrate may be any of those materialstypically used for preparing catalysts, and will preferably comprise aceramic or metal honeycomb structure. Any suitable substrate may beemployed, such as a monolithic substrate of the type having fine,parallel gas flow passages extending therethrough from an inlet or anoutlet face of the substrate, such that passages are open to fluid flowtherethrough (referred to as honeycomb flow through substrates). Thepassages, which are essentially straight paths from their fluid inlet totheir fluid outlet, are defined by walls on which the catalytic materialis coated as a washcoat so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels, which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular, etc. Such structures may containfrom about 60 to about 300 or more gas inlet openings (i.e., cells) persquare inch of cross section. FIG. 13 shows an exemplary substrate, awallflow catalytic honeycomb filter 30, having alternately blockedchannels. FIG. 14 depicts a partial sectional view showing alternatelyblocked channels 52. Alternate channels are plugged at the inlet withinlet plugs 58 and at the outlet with outlet plugs 60 to form opposingcheckerboard patterns at the inlet 54 and outlet 56. A gas stream 62enters through the unplugged channel inlet 64, is stopped by outlet plug60 and diffuses through channel walls 53 to the outlet side 66. The gascannot pass back to the inlet side walls 53 because of inlet plugs 58.The inlet side walls 53 can be coated with a porous catalystcomposition.

The substrate can also be a wall-flow filter substrate, where thechannels are alternately blocked, allowing a gaseous stream entering thechannels from one direction (inlet direction), to flow through thechannel walls and exit from the channels from the other direction(outlet direction). Either NSR and/or SCR catalyst composition can becoated on the wall-flow filter. If such substrate is utilized, theresulting system will be able to remove particulate matters along withgaseous pollutants. The wall-flow filter substrate can be made frommaterials commonly known in the art, such as cordierite or siliconcarbide.

The ceramic substrate may be made of any suitable refractory material,e.g., cordierite, cordierite-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, -alumina, an aluminosilicate andthe like.

The substrates useful for the catalysts of the present invention mayalso be metallic in nature and be composed of one or more metals ormetal alloys. The metallic substrates may be employed in various shapessuch as corrugated sheet or monolithic form. Preferred metallic supportsinclude the heat resistant metals and metal alloys such as titanium andstainless steel as well as other alloys in which iron is a substantialor major component. Such alloys may contain one or more of nickel,chromium and/or aluminum, and the total amount of these metals mayadvantageously comprise at least 15 wt. % of the alloy, e.g., 10-25 wt.% of chromium, 3-8 wt. % of aluminum and up to 20 wt. % of nickel. Thealloys may also contain small or trace amounts of one or more othermetals such as manganese, copper, vanadium, titanium and the like. Thesurface or the metal substrates may be oxidized at high temperatures,e.g., 1000° C. and higher, to improve the resistance to corrosion of thealloys by forming an oxide layer on the surfaces the substrates. Suchhigh temperature-induced oxidation may enhance the adherence of therefractory metal oxide support and catalytically promoting metalcomponents to the substrate.

In alternative embodiments, one or both of the NSR or SCR catalystcompositions may be deposited on an open cell foam substrate. Suchsubstrates are well known in the art, and are typically formed ofrefractory ceramic or metallic materials.

Alternative Embodiments of the Emission Treatment System

While the emissions treatment system as shown in FIG. 1A can be usedalone, optionally the system can contain a diesel oxidation (DOC)catalyst 4A upstream of the NSR catalyst 2 and/or downstream (as 4B) ofthe SCR catalyst 3 as shown in the system 10B in FIG. 1B. DOCcompositions are well known in the art and may comprise base metals(e.g., ceria) and/or platinum group metals as catalytic agents.

In an upstream position (i.e., as 4A) the DOC provides severaladvantageous functions. The catalyst serves to oxidize unburned gaseousand non-volatile hydrocarbons (i.e., the soluble organic fraction of thediesel particulate matter) and carbon monoxide to carbon dioxide andwater. Removal of substantial portions of the SOF using the DOCcatalyst, in particular, assists in preventing too great a deposition ofparticulate matter on the NSR and SCR catalysts. In another function, asubstantial proportion of the NO of the NOx component is oxidized to NO₂in the oxidation catalyst. Increased proportions of NO₂ in the NOxcomponent facilitate the trapping and catalytic functions of the NSRcatalyst as compared to NOx mixtures containing smaller proportions ofNO₂, as NO₂ is generally considered to be a more reactive species thanNO.

In configurations where the DOC is located downstream of the SCRcatalyst, the DOC not only serves to combust unburned hydrocarbon and COas described above, but also serves as an ammonia slip oxidationcatalyst to prevent any unreacted NH₃ from venting to the atmosphere,especially with DOC compositions containing platinum group metalcomponents.

In certain embodiments of the invention, the DOC is coated on a sootfilter, for example, a wall flow filter to assist in the removal of theparticulate material in the exhaust stream, and, especially the sootfraction (or carbonaceous fraction) of the particulate material. TheDOC, in addition to the other oxidation function mentioned above, lowersthe temperature at which the soot fraction is oxidized to CO₂ and H₂O.As soot accumulates on the filter, the catalyst coating assists in theregeneration of the filter. Although the soot filter may be locateddownstream of the SCR catalyst it is preferred that the catalyzed sootfilter be located upstream of the NSR catalyst to minimize or preventfouling of the NSR catalyst and the SCR catalyst downstream withparticulate material.

The following examples further illustrate the present invention, but ofcourse, should not be construed as in any way limiting its scope.

EXAMPLE 1 Preparation of Catalyst Substrates (NSR Catalysts)

Catalyst A

The substrate consisted of cordierite. The bottom layer consisted of 75g/ft³ platinum, 0.42 g/in³ BaO, 0.05 g/in³ ZrO₂ and 1.83 g/in³ of asupport consisting of CeO₂-coated Al₂O₃. This support was prepared bythe incipient wetness technique to allow 10 wt. % of CeO₂ on the surfaceof the Al₂O₃. The support material was impregnated with aminesolubilized platinum hydroxide to achieve the desired loading. Thepowder was then milled, in the presence of water, such that 90% of theparticles had a particle size below 10 micrometers (i.e., d₉₀<10 μ),thereby resulting in a high-solids slurry. During the milling process,barium acetate and zirconyl acetate dissolved in water were also addedduring the milling process.

The slurry of the bottom layer was then wash-coated on the cordieritesubstrate and the coated substrate was then dried at 110° C. for aboutone hour. Thereafter, the dried coated substrate was calcined by heatingin a stream of flowing air at 450° C. for one hour.

The top layer slurry was then wash-coated on the surface of the bottomlayer, dried at 110° C. for about one hour and thereafter calcined byheating at 450° C. for one hour. The slurry for the top layer wasprepared in the same manner as that described above for the bottomlayer. The top layer consisted of 65 g/ft³ platinum, 10 g/ft³ rhodium,0.08 g/in³ BaO, 0.03 g/in³ ZrO₂ and 1.20 g/in³ of a support consistingof CeO₂-coated Al₂O₃ (10 wt. % CeO₂). The platinum and rhodiumcomponents were sequentially impregnated on the support material. Thepowder was then milled, in the presence of water, such that 90% of theparticles had a particle size below 10 micrometers (i.e., d₉₀<10 μ),thereby resulting in a high-solids slurry. During the milling process,barium acetate and zirconyl acetate dissolved in water were also addedduring the milling process.

Catalyst B

Catalyst B is a bi-layer catalyst composite that was prepared in thefollowing manner: The substrate was cordierite. The bottom layerconsisted of 75 g/ft³ platinum, 0.42 g/in³ BaO, 0.05 g/in³ ZrO₂ and 1.83g/in³ of a support consisting of TiO₂-coated Al₂O₃. This support wasprepared by the chemical vapor deposition of 10 wt. % of TiO₂ on thesurface of the Al₂O₃. The support material was impregnated with aminesolubilized platinum hydroxide to achieve the desired loading. Thepowder was then milled, in the presence of water, such that 90% of theparticles had a particle size below 10 micrometers (i.e., d₉₀<10 μ),thereby resulting in a high-solids slurry. During the milling process,solutions of barium acetate and zirconyl acetate dissolved in water werealso added during the milling process.

The slurry of the bottom layer was then wash-coated on the cordieritesubstrate and the coated substrate was then dried at 110° C. for aboutone hour. Thereafter, the dried coated substrate was calcined by heatingat 450° C. for one hour.

The top layer slurry was then wash-coated on the surface of the bottomlayer, dried at 110° C. for about one hour and thereafter calcined byheating at 450° C. for one hour. The slurry for the top layer wasprepared in the same manner as that described above for the bottomlayer. The top layer consisted of 65 g/ft³ platinum, 10 g/ft³ rhodium,0.08 g/in³ BaO, 0.03 g/in³ ZrO₂ and 1.20 g/in³ of a support consistingof TiO₂-coated Al₂O₃. The platinum and rhodium components weresequentially impregnated on the support. The powder was then milled, inthe presence of water, such that 90% of the particles had a particlesize below 10 micrometers (i.e., d_(90<10) μ), thereby resulting in ahigh-solids slurry. During the milling process, barium acetate andzirconyl acetate dissolved in water were also added during the millingprocess.

Catalyst C

Catalyst C is a single-layer catalyst composite, i.e., only one layerwas deposited on a cordierite substrate. This single layer was preparedin the same manner as described above for the layers for Catalyst A. Thesingle layer consisted of 150 g/ft³ platinum, 0.40 g/in³ BaO, 0.2 g/in³Fe₂O₃ and 2.4 g/in³ of Al₂O₃ support. The composite was prepared bysequential impregnation of the support using iron nitrate, bariumacetate and amine solubilized platinum hydroxide.

Lean Aging Conditions for Catalyst A, B and C

Lean aging of the catalysts was conducted by exposing the catalyst to astream of air/steam (10% steam) at an inlet temperature of 700° C. for 4hours with a gas hourly space velocity of 30,000 h⁻¹.

EXAMPLE 2 Performance Testing for Emissions Treatment Systems HavingCatalyst A as an NSR Catalyst; Performance as a Function of COConcentration in a Rich Feed

In this example, the NOx conversion, NH₃ and N₂ formation weredetermined as a function of the CO concentration in the rich pulse(i.e., in the rich gaseous stream). These determinations were firstconducted with a system having in serial arrangement Catalyst A as theupstream substrate and a blank flow through monolith substrate as thedownstream substrate. This system is referred to herein as “System I.”

A second system that was evaluated had in a serial arrangement, CatalystA as the upstream substrate and SCR Catalyst D as the downstreamsubstrate, which is referred to herein as “System II.” SCR catalyst Dcontained a catalyst composition with 1.8 g/in³ iron exchanged Betazeolite with 4 wt. % ZrO₂ binder that was coated onto a flow throughmonolith substrate.

Performance tests were conducted with an alternating lean and rich feed,with 50 seconds lean period and 5 seconds rich period. The lean feedconsisted of 10% O₂, 10% H₂O, 5% CO₂ and 200 ppm NO. The rich feedconsisted of 0.8% HC (as C₃H₆), 1% O₂, 10% H₂O, 5% CO₂, 200 ppm NO andalternately, 0.5, 1, 2, 3 or 4% CO. The systems were evaluated attemperatures of 200, 250, 300, 350 and 450° C. with a GHSV of 60,000h⁻¹. Once the performance stabilized at a given temperature, data werecollected for a period of 10 minutes. NO and NO₂ concentrations (andtherefore NOx concentration) and NH₃ formation were determined byFourier transform infrared spectroscopy (FTIR). N₂ formation wasdetermined by calculation as the remaining nitrogen species. The NOxconcentrations downstream of the catalyst were compared with thoseupstream of the system. The relative disappearance of NOx concentration,formation of NH₃ and N₂ were expressed in percentage, were calculatedthroughout the data collection period at the rate of 1/second.

The instantaneous NOx conversions, NH₃ formation and N₂ formation werethen averaged and plotted against the catalyst inlet temperature,resulting in the graphs in FIGS. 2-4 for System I, and FIGS. 5-7 forSystem II. As can be seen by comparing FIGS. 2 and 5 the total NOxconversion is higher for System II than system I when the rich feedcontains 2% or higher CO at inlet temperatures below 300° C. As can beseen for both Systems I and II the concentration of the NH₃ produced wasproportional to the concentration of CO in the rich feed. The NH₃breakthrough at the outlet of System II is much lower than that ofSystem I at any rich CO level (FIGS. 3 and 6). This suggests that a partof the NH₃ produced over the NSR catalyst is consumed on the downstreamSCR catalyst. As shown in FIGS. 4 and 7, the NH₃ consumption over theSCR catalyst contributes to the additional N₂ formed. Only N₂ formationis considered true NOx reduction. When the rich CO concentration ishigher than 2%, the N₂ formation over System II is much higher comparedto those of System I throughout the temperature range. This effect isespecially pronounced at low temperatures, <300° C., which makes thissystem very appealing for light-duty diesel applications where theexhaust temperature is typically low.

EXAMPLE 3 Performance Testing for Emissions Treatment Systems HavingCatalyst A as an NSR Catalyst; Performance as a Function of Rich FeedTiming

In this example, the NOx conversion, NH₃ and N₂ formation weredetermined as a function of the rich feed timing. These determinationswere first conducted with both System I and System II. System I and IIare described in Example 2.

Performance tests were conducted with an alternating lean and rich feed.The lean feed consisted of 10% O₂, 10% H₂O, 5% CO₂ and 200 ppm NO. Therich feed consisted of 0.8% HC (as C₃H₆), 1% O₂, 10% H₂O, 5% CO₂, 200ppm NO and 4% CO. The systems were evaluated at temperatures of 200,250, 300, 350 and 450° C. with a GHSV of 60,000 h⁻¹. In this test, thelength of the rich pulse (rich period) was 1, 3 or 5 seconds while thelean period was always 50 seconds. Once the performance stabilized at agiven temperature, data were collected for a period of 10 minutes. NOand NO₂ concentrations (and therefore NOx concentration) and NH₃formation were determined by FTIR spectroscopy. N₂ formation wasdetermined by calculation as the remaining nitrogen species. The NOxconcentrations downstream of the catalyst were compared with thoseupstream of the system. The relative disappearance of NOx concentration,formation of NH₃ and N₂ were expressed in percentage, were calculatedthroughout the data collection period at the rate of 1/second.

The instantaneous NOx conversions, NH₃ formation and N₂ formation werethen averaged and plotted against the catalyst inlet temperature.

Increasing rich timing has a similar effect to increasing the rich pulseCO concentration. The total NOx conversion increases with increasingrich timing for both System I and System II. However, the NOx conversionover system II is higher than that of System I at <300° C. A morestriking comparison can be made between the NH₃ formed at the outlet ofthe systems. The NH₃ formation over System I is more than twice of thatover System II at all rich timing (see FIGS. 8 and 10). Again, thisindicates that System II consumes NH₃ for the SCR reaction. This isevidenced by that fact that the N₂ formed by System II is much higherthan that of System I at all rich timings (see FIGS. 9 and 11). Forexample, the N₂ formation is 57% over System II at 250° C. with 5 s richwhile that for System I is only 20% under identical conditions.

EXAMPLE 4 Performance Testing for Emissions Treatment Systems HavingCatalyst A as an NSR Catalyst; Performance with a Rich Feed Near theStoichiometric Point

In this example, the NOx conversion, NH₃ and N₂ formation weredetermined with a rich feed that was near the stoichiometric point.These determinations were conducted with both System I and II, whichsystems are described in Example 2.

Performance tests were conducted with an alternating lean and rich feedwith 50 seconds lean period and 5 seconds rich period. The lean feedconsisted of 10% O₂, 10% H₂O, 5% CO₂ and 200 ppm NO. The rich feedconsisted of 1% O₂, 10% H₂O, 5% CO₂, 200 ppm NO and 2% CO. The λ for therich feed was 1.0 (This feed is, in fact, not rich by definition, but isonly rich relative to the lean feed used in this experiment.). Thesystems were evaluated at temperatures of 200, 250, 300, 350 and 450° C.with a GHSV of 60,000 h⁻¹. Once the performance stabilized at a giventemperature, data were collected for a period of 10 minutes. NO and NO₂concentrations (and therefore NOx concentration) and NH₃ formation weredetermined by FTIR spectroscopy. N₂ formation was determined bycalculation as the remaining nitrogen species. The NOx concentrationsdownstream of the catalyst were compared with those upstream of thesystem. The relative disappearance of NOx concentration, formation ofNH₃ and N₂ were expressed in percentage, were calculated throughout thedata collection period at the rate of 1/second.

We have found that when the λ value in a rich pulse was 1.0, no NH₃ wasformed on either system. Therefore, a necessary condition for NH₃formation is the presence of rich environment. As a consequence, thereis no appreciable difference between these two systems in NOxconversion, N₂ or NH₃ formation.

EXAMPLE 5 Comparative Performance Testing for NSR Catalysts; NOxConversion and NH₃ formation with Catalysts A, B and C

In this example, NH₃ formation was compared among three NSR catalysts,Catalysts A, B and C, under identical test conditions. Performance testswere conducted with an alternating lean and rich feed with 50 secondslean period and 5 seconds rich period. The lean feed consisted of 10%O₂, 10% H₂O, 5% CO₂ and 500 ppm NO. The rich feed consisted of 1% O₂,10% H₂O, 5% CO₂, 500 ppm NO and 4% CO. The λ for the rich feed was about0.94 The systems were evaluated at temperatures of 200, 250, 300, 350and 450° C. with a GHSV of 30,000 h⁻¹. Once the performance stabilizedat a given temperature, data were collected for a period of 10 minutes.NO and NO₂ concentrations (and therefore NOx concentration) and NH₃formation were determined by Fourier transform infrared spectroscopy(FTIR). The NOx concentrations downstream of the catalyst were comparedwith those upstream of the system. The relative disappearance of NOx,formation of NH₃ and N₂ were expressed in percentage, were calculatedthroughout the data collection period at the rate of 1/second.

The instantaneous NH₃ formation was then averaged and plotted againstthe catalyst inlet temperature, resulting in the graph in FIG. 12. Ascan be seen in the figure, the NH₃ formation is higher for Catalyst C attemperatures below 300° C., and Catalyst B produces more NH₃ at >300° C.Catalyst A forms the lowest level of NH₃, which centers at 250° C. Itappears that the oxygen storage component present in Catalyst A,comparing to Catalyst B, reduces the NH₃ formation at high temperatures.Presumably, the oxygen storage component becomes reactive at hightemperatures and oxidizes the NH₃ formed to NO and possibly to otherN-containing species. It is also apparent that iron oxide componentpresent in Catalyst C promotes NH₃ formation.

While this invention has been described with an emphasis upon preferredembodiments, it will be obvious to those of ordinary skill in the artthat variations in the preferred devices and methods may be used andthat it is intended that the invention may be practiced otherwise thanas specifically described herein. Accordingly, this invention includesall modifications encompassed within the spirit and scope of theinvention as defined by the claims that follow.

1. An emissions treatment system for an exhaust stream, comprising: aNOx storage reduction (NSR) catalyst comprising a NOx sorbent at aconcentration of at least 0.1 g/in³ and a platinum group metal componentdispersed on a refractory metal oxide support; and, an SCR catalystdisposed downstream of the NSR catalyst, wherein the NSR catalystfurther comprises an oxygen storage component selected from one or moreoxides of cerium and praseodymium in an amount less than 0.5 g/in³. 2.The emissions treatment system of claim 1, wherein the platinum groupmetal components of the NSR catalyst are selected from the groupconsisting of platinum, palladium, rhodium components and mixturesthereof
 3. The emissions treatment system of claim 1, wherein the oxygenstorage component is ceria.
 4. The emissions treatment system of claim1, wherein the NOx sorbent is at least one metal oxide selected from theoxides of lithium, sodium, potassium, cesium, magnesium, calcium,strontium, barium, lanthanum and neodymium.
 5. The emissions treatmentsystem of claim 1, wherein there is less than 0.35 g/in³ of the oxygenstorage component present in the NSR catalyst.
 6. The emissionstreatment system of claim 1, wherein amount of the oxygen storagecomponent present in the NSR catalyst is in the range of from about 0.05g/in³ to about 0.5 g/in³.
 7. The emissions treatment system of claim 1,wherein the refractory metal oxide of the NSR catalyst is selected fromthe group consisting of alumina, titania, zirconia, zeolites, ceriadispersed on alumina, titania dispersed on alumina and silica onalumina.
 8. The emissions treatment system of claim 1, wherein the SCRcatalyst comprises at least one component is selected from the groupconsisting of zeolite component, V₂O₅, WO₃ and TiO₂.
 9. The emissionstreatment system of claim 1, wherein the SCR catalyst comprises azeolite component.
 10. The emissions treatment system of claim 9,wherein the SCR catalyst comprises a zeolite ion exchanged with ionsselected from the group consisting of iron and copper.
 11. The emissionstreatment system of claim 10, wherein the SCR catalyst comprises azeolite ion exchanged with iron.
 12. The emissions treatment system ofclaim 1, wherein the NSR catalyst comprises at least two catalyst layersdeposited on a substrate.
 13. The emission treatment system of claim 12,wherein the two layers comprise a top layer and a bottom layer, and theNOx sorbent in the bottom layer is present in an amount greater thanabout 0.3 g/in³ and that the NOx sorbent in the top layer is present inthe amount of 0.0 to less than about 0.3 g/in³.
 14. The emissionstreatment system of claim 1, wherein the NSR catalyst is a singlecatalyst layer deposited on a substrate.
 15. The emissions treatmentsystem of claim 1, further comprising a diesel engine or a lean burngasoline engine.
 16. The emissions treatment system of claim 1, whereinthe system further comprises a controller to periodically lower theair/fuel ratio in the exhaust stream upstream of the NSR catalyst. 17.The emissions treatment system of claim 16, wherein the controllercomprises an injector that periodically meters a reducing agent selectedfrom at least one of a hydrocarbon fuel, carbon monoxide and hydrogeninto the exhaust stream upstream of the NSR catalyst to form a richgaseous stream.
 18. The emissions treatment system of claim 1, whereinthe SCR catalyst is disposed on a ceramic or metallic honeycomb flowthrough substrate.
 19. The emissions treatment system of claim 1,wherein the NSR catalyst is disposed on a honeycomb flow throughsubstrate.
 20. The emissions treatment system of claim 1, furthercomprising an upstream diesel oxidation catalyst upstream of the NSRcatalyst and/or a downstream oxidation catalyst downstream of the SCRcatalyst.
 21. The emissions treatment system of claim 20, furthercomprising the upstream diesel oxidation catalyst upstream of the NSRcatalyst.
 22. The emissions treatment system of claim 20, furthercomprising the downstream diesel oxidation catalyst downstream of theSCR catalyst.
 23. The emissions treatment system of claim 20, furthercomprising the upstream diesel oxidation catalyst upstream of the NSRcatalyst and the downstream oxidation catalyst downstream of the SCRcatalyst.
 24. The emissions treatment system of claim 1, wherein thereis a soot filter disposed upstream of the NSR or between the NSRcatalyst and the SCR catalyst.
 25. The emission treatment system ofclaim 1, wherein the NSR is present in an amount sufficient to sorb NOxin a gaseous stream and to reduce the sorbed NOx to NH₃ upon contactwith the gaseous stream.
 26. The emission treatment system of claim 1,wherein the NOx sorbent is present in an amount from about 0.1 g/in³to1.0 g/in³.
 27. The emission treatment system of claim 1, wherein the NOxsorbent is present in an amount of at least about 0.3 g/in³.